When a human skeletal joint is damaged, whether as a result of an accident or illness, a prosthetic replacement of the damaged joint may be necessary to relieve pain and to restore normal use to the joint. Typically the entire joint is replaced by means of a surgical procedure that involves removal of the ends of the corresponding damaged bones and replacement of these ends with prosthetic implants. This replacement of a native joint with a prosthetic joint is referred to as a primary total-joint arthroplasty.
For a damaged human knee, the total knee is commonly replaced with prosthetic components shaped to replace portions of the distal femur, proximal tibia and patella. Prosthetic components for use in replacing the distal femur are shaped to replace the articulating surfaces (shown at 21, 23 in FIG. 1) of the medial condyle (shown at 20 in FIG. 1), lateral condyle (shown at 22 in FIG. 1) and trochlea, and prosthetic components for use in replacing the proximal tibia are shaped to replace the tibial plateau. Commonly, the tibial component is two piece: one piece is affixed to the bone and the other piece is a bearing with concave surfaces receiving the femoral condyles. Frequently, a portion of the patella is also replaced with a prosthetic component as part of the total knee replacement.
In some patients, only a portion of the knee is damaged or injured. For such patients, individual compartments of the knee may be replaced. For example, the medial or lateral compartment of the knee may be replaced with uni-condylar components that replace the articulating surface of one condyle of the distal femur and one side of the tibial plateau. The patellofemoral compartment may be replaced with a femoral component that replaces a portion of the trochlea and a patellar component that replaces part of the patella. In some instances, two or three unicompartmental components are implanted together in one joint; for example, two sets of uni-condylar components could be implanted together to replace the articulating surfaces of both the medial and lateral sides of the tibio-femoral joint, a trochlear component (and patellar component) and a set of uni-condylar femoral and tibial components could be implanted together, or two sets of uni-condylar components and a trochlear component (and patellar component) could be implanted together. The following journal articles report, among other things: use of patellofemoral components (trochlear component and patellar component) and one or two sets of uni-condylar components, Arciero, Major and Toomey, “Patellofemoral Arthroplasty: A Three-to-Nine Year Follow-Up Study,” 236 Clinical Orthopaedics and Related Research, Vol. 236, Nov. 1, 1988, pages 60-71; and two sets of uni-condylar components, Bourne, Rorabeck, Finlay and Nott, “Kinematic I and Oxford Knee Arthroplasty: A 5-8-year Follow-up Study,” The Journal of Arthroplasty, Vol. 2, No. 4, December, 1987, pages 285-291, and Shoji, D'Ambrosia and Lipscomb, “Failed Polycentric Total Knee Prostheses,” The Journal of Bone and Joint Surgery, Vol. 58-A, No. 6, September 1976, pages 773-777, and Stockley, Douglas and Elson, “Bicondylar St. Georg Sledge Knee Arthroplasty,” Clinical Orthopaedics and Related Research, No. 255, June, 1990, pages 228-234.
Patents and published applications related to uni-condylar knee implant components or patellofemoral implant components include the following: U.S. Pat. No. 3,852,830; U.S. Pat. No. 3,953,889; U.S. Pat. No. 4,034,418; U.S. Pat. No. 4,340,978; U.S. Pat. No. 4,838,891; U.S. Pat. No. 5,871,541; U.S. Pat. No. 6,616,696; and U.S. Pat. No. 6,709,460.
Commercial uni-condylar knee implant components or patellofemoral implant components include the LCS® UNI Unicompartmental Knee System (DePuy Orthopaedics, Warsaw, Ind.), the Preservation™ Uni-Compartmental Knee (DePuy Orthopaedics, Warsaw, Ind.), the LCS® PFJ Prosthesis (DePuy Orthopaedics, Warsaw, Ind.), the Patella MOD III and Patella II (Smith & Nephew/Richards) and the Oxford (Biomet).
When knees are replaced with common total joint prostheses, substantially all of the potential articulating surface of the distal femur is replaced and covered with metal; no native articular cartilage remains exposed in the potential area of articulation. In contrast, when one or more compartments of a knee are replaced with unicompartmental components, substantial areas of native cartilage are not covered by metal, and remain exposed. FIG. 1 illustrates an example of a human femur 10 with an implanted trochlear implant component 11. FIG. 2 illustrates an example of a human femur 10 with an implanted trochlear implant component 11 replacing the articulating surface of the trochlea together with a uni-condylar femoral component 13 replacing the articulating surface of one of the femoral condyles. In FIG. 2, the areas of exposed native tissue include the intercondylar notch 16, and areas 18, 19 of the distal femoral condyles 20, 22 adjacent to the intercondylar notch 16 and an area 24 of the distal femoral condyles 20, 22 lying between the distal portion 27 of the trochlear component 11 and the anterior portion 29 of the uni-condylar femoral component 13. As shown in FIGS. 1-2, the distal portion 26 of the trochlear component 11 generally tapers toward its distal end, which is positioned near or within the intercondylar notch 16.
FIG. 3 illustrates the femur 10 of FIG. 2, shown with a patellar implant component 31 engaging the trochlear component 11. The patellar component 31 includes a bearing surface 33 that bears against a bearing surface 35 of the trochlear component 11. The exposed bearing surface 35 of the illustrated trochlear implant component 11 has two convex surfaces 39, 41 meeting along a groove 43. FIG. 4 illustrates the femur of FIG. 3 with the patellar component 31 positioned with respect to the trochlear component 11 as it would be with the knee in deep flexion. When the knee is in deep flexion, a portion of the patellar component 31 may extend beyond the edges of the distal portion 27 of the trochlear component 11. Such an overhanging portion (shown at 37 in FIG. 4) of the patellar component 31 may contact and rub against the patient's native tissue (such as native tissue indicated at 18, 19 and 24 in FIG. 4) as the knee flexes and extends. This contact may result in painful irritation of the native tissue. This painful irritation could be prevented through use of a total knee prosthesis; however, use of a total knee prosthesis could result in an unnecessary loss of healthy bone tissue. The pain resulting from this irritation could be treated by revising the surgery, replacing the uni-compartmental components 11, 13 with a total knee prosthesis, again resulting in the loss of healthy bone tissue. A need exists for a means for preventing or treating the patient's native tissue near the intercondylar notch without requiring the removal and replacement of healthy tissue.
U.S. Pat. Publication No. 2005/0177242 A1, entitled “Patello-Femoral Prosthesis,” discloses a trochlear component with an intercondylar notch portion with tapered wings extending distally and curved posteriorly. The wings also curve away from each other in the posterior direction. Although the wings provide additional bearing surfaces for the patellar implant component, they may not cover the portions of the femur that potentially contact the patellar prosthesis-bearing surface. In addition, individual patient anatomies may prevent use of such a trochlear implant in all patients.